The present invention is directed generally to an improved method and system for operating an electrically controlled pressurized-fluid brake system for a vehicle.
DE 196 37 484 A1, which is incorporated herein by reference, discloses a pressurized-fluid brake system for a tractor-trailer combination in which the braking pressure in the brake cylinder is adjusted by means of a valve device based on a set-pressure command. The set pressure is transmitted by a pressure-supply conduit of the tractor vehicle to the trailer vehicle coupled thereto. This set pressure represents the intensity of brake actuation by the vehicle operator of the tractor vehicle.
A set-pressure sensor is provided, in or on the trailer vehicle, for sensing the set pressure. The set-pressure sensor delivers set-pressure data representing the set pressure to an electrical controller used to control the pressurized-fluid brake system of the trailer vehicle. The controller controls electrically actuatable valve devices in such a way that braking pressure corresponding to the set pressure is established in the brake cylinder. During normal operation, the feed of pressurized fluid is ensured by the fact that pressurized fluid is obtained from a master cylinder of the pressurized-fluid brake system of the trailer vehicle.
By virtue of the electrical control of braking pressure, the brake system responds faster than is the case in conventional actuation purely by pressurized fluid, and it can be actuated dynamically, since pneumatic dead times can be greatly reduced. As a safeguard against disturbances in the electrical controller, a changeover valve is provided by which the feed of pressurized fluid to the brake cylinder from the master cylinder can be switched to set pressure. In such case, the brake system is actuated purely pneumatically, as in conventional pressurized-fluid brake systems.
In normal operation, when the pressurized-fluid brake system is under electrical control, the set-pressure sensor has an important role in the functional reliability of the brake system, since the data delivered by this sensor are used to control the braking pressure in the brake cylinder and, thus, the deceleration of the vehicle. It is therefore important that the data delivered by the set-pressure sensor be checked for accuracy. To this end, in a known arrangement, there is disposed in a pressurized-fluid path conveying the set pressure, not only the set-pressure sensor but also a pressure switch, which delivers a digital switching signal at a particular pressure threshold. This pressure threshold is selected to be close to the response threshold of the electrical controller, an example being 0.7 bar. The pressure threshold can also be selected within the range of the response pressure of the pressurized-fluid brake, an example being 0.2 bar.
An important function of the pressure switch is to recognize and prevent that dangerous condition in which the set pressure is 0 bar, because the vehicle operator has not actuated the brake system of the vehicle, but, nevertheless, a much higher set pressure is being signaled, because a defect, for example, is causing the set-pressure sensor to deliver erroneous set-pressure data to the controller. To ensure that the controller does not adjust the braking pressure to correspond to incorrect set-pressure data, the controller additionally samples the signal of the pressure switch and, in this way, can recognize that the set pressure is actually lower than the switching threshold of the pressure switch, signifying that the brake cylinder is not to be filled with pressurized fluid and, thus, that braking is not to be initiated.
While the foregoing has already been developed to a highly safe and reliable level, the use of a pressure switch in addition to the set-pressure sensor represents increased equipment complexity. It is also conceivable that the pressure switch could be defective, thus necessitating additional safety and plausibility checks between the signals of the pressure switch and of the set-pressure sensor.
It is desired to provide an improved method and system for operating an electrically controlled pressurized-fluid brake system which achieve high functional reliability of the brake system while minimizing equipment complexity, and, in particular, without the need for a pressure switch.
Generally speaking, in accordance with the present invention, a method and system for operating an electrically controlled pressurized-fluid brake system are provided which improve over prior art methods and systems.
Hereinafter, the term xe2x80x9cbrakexe2x80x9d will be used in its broadest sense to mean not only the components directly involved in vehicle braking, such as brake drums and brake shoes, but also brake-applying devices, such as brake cylinder and transmission parts, such as brake rods, brake shafts and brake cams.
In a preferred embodiment of the present invention, a method is provided for operating an electrically controlled pressurized-fluid vehicle brake system having brake cylinders actuatable by pressurized fluid for actuating the vehicle brakes, braking-pressure sensors for obtaining braking-pressure data representing braking pressure in the brake cylinders, valve devices for influencing braking pressure, the brake cylinders being in communication with the valve devices with a supply pressure and/or a set pressure in order to feed pressurized fluid, and set-pressure sensors for obtaining set pressure data, the braking pressure being adjusted based on the set-pressure data and/or the braking-pressure data. The inventive method includes the steps of checking the set-pressure data by temporarily switching pressurized-fluid feed to the brake cylinders from the supply pressure to the set pressure by actuation of at least one part of the valve devices, and monitoring resulting braking pressure on the basis of the braking-pressure data.
The present invention has the advantage that additional elements such as the pressure switch for checking set-pressure data are not required. Checking can be accomplished purely by means of appropriately programmed software for the electrical controller, and, thus, can be implemented simply and inexpensively. A brake-pressure sensor, which is present in any case and is necessary in any case for precise adjustment of the braking pressure in the brake cylinder, can also be used as a redundant sensor element for checking the set-pressure data.
The present invention can be used in trailer vehicles as well as in tractor vehicles.
In another embodiment of the present invention, all pressure sensors, the electrical controller and also the valve device are structurally integrated into a single compact control module.
According to another embodiment of the present invention, the brake cylinder is vented slightly just before any intended filling of the brake cylinder from the set pressure. In this way, it is possible in simple manner to detect set-pressure data that are too-high. A further advantage of this embodiment is that pressure data that are too-low or too-high can be recognized with a single changeover to set pressure.
If the set-pressure data are too high, the data delivered by the set-pressure sensor are higher than the actual set pressure. Because of the attempt to fill the brake cylinder from set pressure, the braking pressure cannot be raised sufficiently to the level that existed before venting, and in some cases a drop of braking pressure may occur. At the very least, the expected brake-pressure rise to the pressure level adjusted before venting will not occur. The controller can recognize this as defective behavior.
If the set-pressure data are too low, the set-pressure data indicate a pressure lower than the actual set pressure. Prolonged filling of the brake cylinder from set pressure can cause the braking pressure to rise to a pressure level above the pressure established before venting. The controller can also recognize this as defective behavior.
In another embodiment of the present invention, venting is performed within the hysteresis range of the brake; that is, it is kept sufficiently small so that hysteresis does not lead to a change in braking force of the brake despite reduction of braking pressure. Under these conditions, cyclically recurring checks of the set-pressure data obtained by the set-pressure sensor can be performed arbitrarily during a braking action, without resulting in a perceptible reaction from the vehicle.
Accordingly, it is an object of the present invention to provide a method and system for operating an electrically controlled pressurized-fluid brake system which achieve high functional reliability of the brake system.
It is also an object of the present invention to provide a method and system for operating an electrically controlled pressurized-fluid brake system that are cost effective and minimize equipment complexity, in particular, by eliminating the need for a pressure switch.
It is a further object of the present invention to provide a method and system for operating an electrically controlled pressurized-fluid brake system that enable the integration of parts into a single compact module.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The present invention accordingly comprises the various steps and the relation of one or more of such steps with respect to each of the others, and embodies features of construction, combinations of elements, and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.